An ink jet printer is known, such as disclosed in Japanese Patent Application Laid-Open No. 2004-255861, where a recording head equipped including a plurality of nozzles for discharging a plurality of inks is fixedly mounted to the bottom of a substantially box-like shaped head holder functioning as a carriage with the nozzles being exposed at the bottom of the head holder, while a buffer tank including a plurality of ink storage chambers is provided above the recording head. The inks are fed from their respective ink cartridges or ink supply sources via tubes to the buffer tank from which they are delivered to the corresponding nozzles in the recording head before they are ejected from the nozzles for printing on a sheet of recording paper.
In such an ink jet printer, when bubbles of air are produced in the ink, they may block the flow passage in the recording head and thus interrupt discharge of the ink from the nozzle. It is hence essential to remove the bubbles of air from the ink. For the purpose, the bubbles of air are accumulated in the buffer tank by lifting up the bubbles of air in the ink received from the ink cartridge. A plurality of air discharge units are provided in one side of the buffer tank for discharging the air to the outside. FIG. 1 is a side cross sectional view of a conventional buffer tank 100 accompanied with the air discharge units 101. The air discharge unit 101 is provided for each ink including an opening at the lower end 101a, an air discharge passage 102, an air discharge valve 103, and a spring member 104 for producing an elastic force to press and hold the air discharge valve 103 in the closing state. As air has been accumulated in the buffer tank 100, it is discharged from the air discharge passage 102 to the outside by properly controlling the air discharge value 103. The air discharge valve 103 is composed mainly of a valve body 103a acting as a valve for opening and closing the air discharge passage 102 with the help of the elastic force of the spring member 104 and a valve rod 103b extending from the valve body 103a to the lower. The valve body 103a remains pressed by the spring member 104 against the inner wall A of the air discharge passage 102 in the normal state (during printing).
Before discharging the air from the air discharge unit 101, the carriage is moved to the location of an open/close unit 110 provided outside the scanning movement of the carriage for conducting printing. The open/close unit 110 includes air discharge caps 106 for closing the lower openings 101a of the air discharge units 101 and air discharge rods 107 corresponding to the valve rods 103b so as to be operable selectively by a lifting unit for upward and downward movement. Before discharging the air, the air discharge cap 106 is moved to close the lower openings 101a of the air discharge units 101 for permitting to start sucking by the lifting unit. Substantially at the same time, the air discharge rods 107 are lifted up to come into direct contact with and elevate their respective valve rods 103b for producing the valve opening state where the valve bodies 103a depart from the inner wall A.